In the conventional manufacture of polystyrene resin foams, amides, waxes, metal soaps, liquid paraffin and various other additives have been used to reduce the load on the extruding machine and thereby to obtain foams having a satisfactory appearance. However, since the heretofore used additives have no antistatic effect, the resin being extruded from a nozzle may often develop a static buildup on account of the friction with the molding device and can potentially catch fire. Problems have also occurred in the final product, as exemplified by the deposition of dirt, staining, malodor, etc., due to static buildup. Another problem has resulted from the use of inorganic fine powders as void control agents and insufficiently dispersed particles often stay within the extruding machine or form deposits, which are occasionally extruded from the machine to introduce defects in the final product.
The present inventors reviewed various additives with a view to solving the aforementioned problems of the prior art; as a result, they found that all of the problems could be solved by adding 0.01 to 2 parts by weight of an alcoholic ester of a higher fatty acid to 100 parts by weight of a polystyrene based resin.
Higher fatty acid esters of polyhydric alcohols are commonly used as lubricants or antistats in polyvinyl chloride, polyethylene and polypropylene but not in polystyrene resins. Mono-, di- and triglycerides of stearic acid are the most often used of the higher fatty acid esters of polyhydric alcohols Foams that are free from shrinkage can be obtained by using those glycerides in the manufacture of polyethylene foams and this has been well known by being disclosed in JP-B-46-43997, JP-B-46-43998, JP-B-47-33505, JP-B-58-30896, etc. (The term "JP-B" as used herein means an "examiner Japanese patent publication".) As regards polystyrene resin foams, natural fats and oils containing triglycerides as main components have sometimes been used as described in JP-A-60-233135. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)